1. Field of the Invention
This invention relates to a thermodevelopable photographic material, and particularly to a thermodevelopable photographic material containing a compound which inhibits fog caused by heating (development) (to be referred to hereinafter for brevity simply as "fog").
2. Description of the Prior Art
Photographic methods utilizing silver halide have previously gained the most widespread acceptance because they give rise to superior photographic characteristics such as sensitivity and gradation to methods such as electrophotography or diazotype photographic processes. The silver halide photographic materials used in these methods are first developed with a developer after imagewise exposure, and then in order for the developed image not to discolor or fade under normal illumination, or for the undeveloped part (to be referred to hereinafter as background) not to turn black, the materials must be subjected to treatments such as stopping, fixation, rinsing or stabilization. Accordingly, the processing of a photographic material is time-consuming and requires much labor; moreover, the handling of various chemicals involves risks to the human body, and also the contamination of the processing chamber or the hands or garments of the working personnel. It is therefore very desirable to improve the photographic processes which use silver halides so that the processing of the photographic material can be performed in the dry state without using a solution treatment, and the processed image is maintained stable.
Various efforts have been made with a view to achieving this improvement. One of these efforts has resulted in a photographic process using a thermodevelopable photographic material comprising, as a photographic element, a major amount of a silver salt of a long-chain aliphatic carboxylic acid and a catalytic amount of a silver halide as is disclosed in U.S. Pat. Nos. 3,152,904; 3,457,075; 3,635,719; 3,645,739 and 3,756,829 and Japanese Pat. No. 22185/70.
When, for example, a photographic material comprising a composition consisting of a fatty acid-silver salt, a reducing agent and a catalytic amount of a silver halide, as one of the thermodevelopable photographic materials heretofore proposed, is heated, an oxidation-reduction reaction between the fatty acid-silver salt and the reducing agent occurs due to the catalytic action of the silver halide at the exposed portion to form a silver image; however, at the same time, an oxidation-reduction reaction proceeds also at the non-exposed portion to cause undesirable fog.
In an attempt to prevent fog formation, a method has already been proposed in which a compound capable of donating a mercury ion is used. For example, U.S. Pat. No. 3,589,903 discloses that a thermodevelopable material having a reduced tendency toward fog formation can be obtained by incorporating a compound capable of donating a mercury ion into a composition consisting of an organic silver salt, a reducing agent and a catalytic amount of a light-sensitive silver halide. Since in this technique, the compound donating a mercury ion also acts as a light-sensitive silver halide, the use of the light-sensitive silver halide can be omitted.
As is well known, however, mercury compounds are toxic, and the photographic processing operation could be very risky. Furthermore, if unwanted coating solutions containing mercury ions are discarded into rivers or the like, the mercury level builds up in fish and shell-fish, and will cause serious problems upon human consumption. Also when photographic materials containing mercury compounds are re-used, for example, as regenerated paper, the mercury compounds flow out likewise causing problems.
Accordingly, an object of this invention is to provide a thermodevelopable photographic material containing a compound which is not toxic, in contrast to mercury compounds, but possesses a fog inhibiting effect, as with the mercury compounds.
Another object of this invention is to provide a thermodevelopable photographic material whereby the image obtained after development has reduced discoloration by light.
Still another object of this invention is to provide a thermodevelopable photographic material which can give good quality images free from fog and having high image density and good contrast.